.app,html,body{
    width: 100%;
    height: 100%;
}
/* app 开始 */
.app{
    display: flex;
    flex-direction: column;
}
/* header */
header{
    width: 100%;
    height: 1rem;
    background: pink;
}
/* main */
main{
    flex: 1;
    overflow-y: auto;
}
/* footer */
footer{
    width: 100%;
    height: 1rem;
    background: yellow;
}